This invention relates to a beam splitter element and to a photosensor assembly which utilizes such beam splitter element.
Photosensor apparatus, in which a beam of light is directed outwardly from the apparatus to intercept and ultimately detect retroreflective material encountered by the beam, is utilized in a variety of circumstances including the controlling of movement of passenger and freight elevators, automatic warehouse handling systems, etc. It is important with such photosensor apparatus that the apparatus accurately detect the presence of the retroreflective material and that false detections be eliminated or greatly minimized since such false detections could cause erroneous operation of the elevator, automatic warehouse handling system, etc. Such false detection is typically caused by light, referred to as "noise", which finds its way into the apparatus so that the apparatus erroneously interprets such light as having been reflected from retroreflective material.
One of the primary aims in designing photosensor apparatus is to minimize the possibility of light "noise" interferring with the operation of the apparatus. An example of a photosensor designed with this in mind is disclosed in U.S. Pat. No. 3,214,569. This photosensor apparatus utilizes a conventional partial mirror type beam splitter for directing a light beam outwardly of the apparatus and then for directing reflected light towards a photocell. One of the problems with this apparatus is that light is reflected from the back surface of a focusing lens onto the partial mirror which then directs this reflected light to the photocell. This contributes to the light "noise" and thus to the possibility of false operation of the apparatus. Another problem with apparatus such as that described in the above-noted patent is that the partial mirror is quite fragile and thus subject to breakage if the apparatus is jarred or roughly handled. Finally, with apparatus such as that described, so-called bar codes composed of alternating areas of retroreflective and non-reflective material must have fairly large dimensions in order to be "read" by the apparatus. This is so because the " field of view" of the photocell of such apparatus is quite large, generally corresponding to the size of the projected beam of light.